1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In lithographic apparatuses the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatuses employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray. Other proposed radiation types include electron beams and ion beams. These types of beam share with EUV the requirement that the beam path, including the mask, substrate and optical components, be kept in a vacuum. When sub-micron accuracy is required, even very small disturbances can be damaging. In conventional lithographic apparatuses, dynamic heat loads within the apparatus resulting in thermo-mechanical deformations of critical components, such as a reference frame which provides multiple functionalities and requires a highly stable environment and a projection system, continue to cause problems with imaging accuracy. However, these problems are further exacerbated in lithographic apparatuses having a projection beam at shorter wavelengths, such as EUV, because the high vacuum required make it difficult to sink absorbed heat and involve relatively high absorption.
It is an advantage of an embodiment of the present invention to address the problems associated with heat loads in lithographic apparatuses, resulting in thermo-mechanical deformations of critical components.
It is an advantage of an embodiment of the present invention to improve the thermal stability of critical components by reducing the thermal drift of the reference frame and optical components mounted on the reference frame, resulting in a decrease of relative displacements of the measurement system with respect to each other and with respect to the optical system. Furthermore it is an advantage of an embodiment of the present invention to improve the dynamic performance by the thermal shielding of the reference frame and optical system, since the required active thermal conditioning capacity of the reference frame and optical system can be reduced.